1. Field of the Invention
This invention relates to a gear-shifting shock suppressing system for an automatic transmission vehicle.
2. Description of the Prior Art
The automatic transmission for a vehicle typically comprises a torque convertor having a pump impeller, a turbine runner and a stator, a transmission gear mechanism which is connected to the turbine runner of the torque convertor and has a plurality of friction coupling members such as clutches and brakes, and a hydraulic control system having a hydraulic control circuit which controls the condition of application (applied or released) of the friction coupling members under the control of a control means, thereby changing the power transmission path of the transmission gear mechanism so that the transmission gear mechanism shifts to a desired gear-speed according to the running condition of the vehicle.
During the gear-shifting operation, the engine speed changes with change in the gear ratio to cause fluctuation in the transmission output torque whereas the vehicle speed does not substantially change due to the inertia of the vehicle body. The fluctuation in the transmission output torque changes the acceleration of the vehicle body and causes a so-called gear-shifting shock. The gear-shifting shock may be suppressed when the hydraulic pressure applied to the friction coupling members is controlled so that they are smoothly applied or released, but this approach is disadvantageous in that the friction coupling members are kept slipping for a long time, which causes seizing in the friction coupling members and increases wear of them.
Thus it has been proposed to retard the ignition timing so that the engine output is temporarily lowered and the gear-shifting shock is suppressed. See Japanese Unexamined Patent Publication No. 62(1987)-131831, for instance.
However since there is a certain time lag between the time the hydraulic control system outputs a gear-shifting signal and the time the transmission gear mechanism actually shifts to the gear-speed designated by the signal and the gear-shifting shock occurs, the ignition timing must be retarded in time to the occurrence of the gear-shifting shock. Otherwise, the gear-shifting shock can be enhanced or the acceleration performance is adversely affected. The gear-shifting shock suppressing system disclosed in Japanese Unexamined Patent Publication No. 60(1985)-175855, the ignition timing is controlled so that the engine output is lowered a predetermined time after the hydraulic control system outputs the gear-shifting signal. Further, the degree of gear-shifting shock depends upon the kind of gear-shifting (upshift or downshift?), and upon the gear-speeds before and after the gear-shifting, and in order to effectively suppress the gear-shifting shock without adversely affecting the acceleration performance or the like, the ignition timing should be controlled taking into account the degree of gear-shifting. The gear-shifting shock suppressing system disclosed in Japanese Patent Publication No. 63(1988)-14171, the engine output is lowered a predetermined time after the hydraulic control system outputs an upshift signal for a predetermined time which is determined according to the kind of the upshift signal.
However, there has been a problem that the gear-shifting shock cannot be satisfactorily suppressed when the gear-shifting is repeatedly effected in a short time.
For example, if a 3-2 downshift signal is generated a relatively long time after generation of a 4-3 downshift signal, the transmission gear mechanism is once shifted to third and thereafter to second. Accordingly, the gear-shifting shock occurs twice. However if the time between the generation of the 4-3 downshift signal and the generation of the 3-2 is extremely short, the transmission gear mechanism is substantially directly shifted to second and the gear-shifting shock is substantially equivalent to the gear-shifting shock accompanying the 4-2 downshift. That is, the time the gear-shifting shock occurs and the degree of the gear-shifting shock are largely affected by the interval between the gear-shifting signals. Accordingly, if the engine output is controlled simply based on the 4-3 downshift signal or the 3-2 downshift signal, satisfactory shock suppressing effect cannot be expected.